230 System Administration disk pack.) According to Andy Tannenbaum, “If Unix crapped on your rk05, you’d write to Dennis for another.”)1 Most Unix systems come equipped with a special partition called the “swap partition” that is used for virtual memory. Early Unix didn’t use the file system for swapping because the Unix file system was too slow. The problem with having a swap partition is that the partition is either too small, and your Unix craps out when you try to work on problems that are too large, or the swap partition is too large, and you waste space for the 99% of the time that you aren’t running 800-megabyte quantum field dynamics simulations. There are two simple rules that should be obeyed when partitioning disks:2 1. Partitions must not overlap. 2. Each partition must be allocated for only one purpose. Otherwise, Unix will act like an S&L and start loaning out the same disk space to several different users at once. When more than one user uses “their” disk space, disaster will result. In 1985, the MIT Media Lab had a large VAX system with six large disk drives and over 64 megabytes of memory. They noticed that the “c” partition on disk #2 was unused and gave Unix permission to use that partition for swapping. A few weeks later the VAX crashed with a system panic. A day or two after that, somebody who had stored some files on disk #2 reported file cor- ruption. A day later, the VAX crashed again. The system administrators (a group of three undergraduates) eventually discovered that the “c” partition on disk #2 overlapped with another parti- tion on disk #2 that stored user files. This error lay dormant because the VAX had so much memory that swap- ping was rare. Only after a new person started working on a large image- processing project, requiring lots of memory, did the VAX swap to the “c” partition on disk #2. When it did, it corrupted the file system—usually resulting in a panic. 1Andy Tannenbaum, “Politics of UNIX,” Washington, DC USENIX Conference, 1984. (Reprinted from a reference in Life With Unix, p. 13) 2Indeed, there are so many problems with partitioning in Unix that at least one ven- dor (NeXT, Inc.) recommends that disks be equipped with only a single partition. This is probably because NeXT’s Mach kernel can swap to the Unix file system, rather than requiring a special preallocated space on the system’s hard disk.

Disk Partitions and Backups 231 A similar problem happened four years later to Michael Travers at the Media Lab’s music and cognition group. Here’s a message that he for- warded to UNIX-HATERS from one of his system administrators (a posi- tion now filled by three full-time staff members): Date: Mon, 13 Nov 89 22:06 EST From: saus@media-lab.mit.edu Subject: File Systems To: mt@media-lab.mit.edu Mike, I made an error when I constructed the file systems /bflat and /valis. The file systems overlapped and each one totally smashed the other. Unfortunately, I could find no way to reconstruct the file sys- tems. I have repaired the problem, but that doesn’t help you, I’m afraid. The stuff that was there is gone for good. I feel bad about it and I'm sorry but there’s nothing I can do about it now. If the stuff you had on /bflat was not terribly recent we may be able to get it back from tapes. I’ll check to see what the latest tape we have is. Down and Backups Disk-based file systems are backed up regularly to tape to avoid data loss when a disk crashes. Typically, all the files on the disk are copied to tape once a week, or at least once a month. Backups are also normally per- formed each night for any files that have changed during the day. Unfortu- nately, there’s no guarantee that Unix backups will save your bacon. From: bostic@OKEEFFE.CS.BERKELEY.EDU (Keith Bostic) Subject: V1.95 (Lost bug reports) Date: 18 Feb 92 20:13:51 GMT Newsgroups: comp.bugs.4bsd.ucb-fixes Organization: University of California at Berkeley We recently had problems with the disk used to store 4BSD system bug reports and have lost approximately one year’s worth. We would very much appreciate the resubmission of any bug reports sent to us since January of 1991. The Computer Systems Research Group.1